Flashcards in Calculations Deck (16):
How do you convert stone to kg?
pounds x 6.35 (14 pounds in a stone)
How do you convert inches to cm?
inches x 2.54 (12 inches in a foot)
Where do you find amount of creams needed (eg both hands for 1 week)
Suitable quantities for prescribing - skin conditions management
Where do you find info on COCP?
Combined hormonal contraception
What is bioavailability?
Fraction of dose reaching general circulation as active form
Available dose = administered dose x f
What is the volume of distribution?
Volume which a drug is distributed in body
5 litres if confined to plasma
10-15L extracellular water
40L total body water
Fixed characteristic of drug, L or L/kg
Can you work out dose needed for a desired plasma concentration?
Yes if the vol of distribution (Vd) is known
Plasma conc = dose/Vd
What is elimination?
Metabolism + excretion
What is zero order elimination?
Fixed rate, linear graph
eg 1 unit alcohol eliminated in 1hr
What is 1st order elimination?
Exponential elimination curve
Due to fixed clearance rate through kidney, dependent on flow and conc
Higher drug conc, faster elimination rate
What is clearance?
Vol of blood from which the drug is cleared per unit time (ml/min)
NOT amount of drug
How can rate of elimination be calculated from clearance?
Rate of elimination (mg/h) = clearance (L/h) x plasma conc (mg/L)
Cancel out the L
How can you calculate rate of drug infusion needed?
For steady state:
￼Rate of drug infusion (mg/h)= Rate of drug elimination (mg/h)
= Clearance (L/h) x steady state plasma conc (mg/L)
Is the calculation of maintenance oral doses different from infusions?
Final answer the same (rate of dosing = rate of elimination = clearance x steady state conc)
But equation different due to intermittent dosing
(Oral dose x bioavailability)/time interval between doses = clearance x steady state conc
How long does it take to reach the steady state plasma concentration desired?
3-5 x half life